1. Field of the Invention
The present invention relates to dies for extruding thermoplastic sheet material. More particularly, the invention relates to dies wherein the leading edge of at least one die lip is beveled to a certain critical angle.
2. Description of the Prior Art
Dies for extruding molten thermoplastic material in sheet form are well known in the prior art. It is also know in the art that a controlled degree of microsurface roughness is desirable in thermoplastic sheet materials in order to minimize the tendency for the thermoplastic sheets to block, i.e., to adhere when they are in contact with each other as in rolled form or in stacks of cut sheet. U.S. Pat. No. 2,489,951 teaches die lips which are grooved in the direction of extrusion in order to prepare sheets having a controlled degree of microsurface roughness. While the invention in U.S. Pat. No. 2,489,951 provides a controlled degree of microsurface roughness a need exists in the art for improved apparatus and methods for imparting an even greater degree of microsurface roughness to extruded thermoplastic sheets. This is especially so in regard to the thermoplastic sheets used to prepare laminated safety glass. Improvements in the handling of plastic sheet material and the fabrication of laminated safety glass have created a need for plastic sheet material with less tendency to block itself or to adhere prematurely to the glass during the shaping and assembly operations.